Not Applicable.
Not Applicable.
The present invention is generally directed to a method for attaching first and second members, and subsequently adjusting the shape and/or dimensions of at least one of the members. More particularly, the present invention is directed to a method of attaching one or more cam lobes and other individual camshaft elements, such as, for example, sprockets, bearing races and gears, to a camshaft tube at desired axial and angular orientations about the tube, and subsequently adjusting the shape and/or dimensions of at least one of the attached elements. In this way, the cam lobes and components may be adjusted to meet dimensional tolerances necessary for proper assembly of the camshaft into a combustion engine or other apparatus.
The method of the present invention may be utilized in applications wherein it is desirable to attach first and second members and subsequently adjust the shape and/or dimensions of at least one of the members. Examples of such applications include the fabrication of camshafts for internal combustion engines and other applications and the assembly of complex gear forms on rotatable shafts. The present method is particularly useful when applied in the fabrication of camshafts wherein cam lobes and, possibly, other camshaft elements such as sprockets, bearing races, and gears, are installed in precise locations on the outer surface of a camshaft tube. Camshafts fabricated according to the method of the invention may be used to engage push rods, lifters, impellers, rotors, gears, pulleys or other movable members.
Camshafts for piston-driven internal combustion engines typically include several cam lobes with lobe-shaped outer surfaces that operate to move push rods, lifters, or other movable members in a precise pattern. As the camshaft rotates, the cam lobes must engage the movable members at proper positions and with proper timing. Therefore, the cam lobes must be positioned on the camshaft at precise relative axial positions and angular orientations.
A camshaft 10 of a typical construction adapted for use in an internal combustion engine is depicted in FIG. 1. The camshaft 10 generally includes a camshaft tube 11. Several cam lobes 12 are affixed to the outer surface of the camshaft tube 11. Other camshaft components such as, for example, gear 16, may be affixed to the outer surface of the camshaft tube 11. Although generically referred to herein as a xe2x80x9ccamshaft tubexe2x80x9d, that element, although typically hollow, need not be cylindrical and may have any overall shape and uniform or non-uniform cross-section suitable for receiving and rotating the several cam lobes and other camshaft components. Accordingly, xe2x80x9ccamshaft tubexe2x80x9d is used herein to refer generally to the central rotating component of a camshaft to which the cam lobes are affixed and is not limited to any particular cylindrical or non-cylindrical configuration.
The lobe-shaped region of each cam lobe has a predetermined shape and is dimensioned to accurately control movement of the movable member it engages. More specifically, the profile of the cam lobe 12, and particularly the shape and dimensions of the lobe-shaped region 13, are such that as the camshaft tube 11 rotates, the motion of the cam lobe 12 imparts a precise rocking or reciprocating motion to the movable member it engages. In FIG. 1, for example, the movable members illustrated adjacent a cam lobe 12 are a lifter 14 and a push rod 15. As the camshaft 10 rotates, the surface shape and dimensions of each cam lobe 12, along with their various angular and axial positions along the length of the camshaft tube 11, work in conjunction to properly move the push rods 15 of the engine in a desired pattern and timing. This synchronized motion ensures that the intake and exhaust valves of all engine cylinders operate correctly.
Camshafts combining a camshaft tube and several cam lobes traditionally were manufactured as a single component by casting or forging. This method of fabrication is time-consuming and expensive, and produces camshafts with limited dimensional accuracy. Therefore, extensive grinding and/or polishing is required to shape the individual cam lobes and other camshaft components and appropriately adjust the shape and dimensions of the surfaces of each of the components. Absent such extensive finishing work, the cam lobes would not properly engage their associated movable members. Forged or cast camshafts are necessarily composed of material of a substantially homogenous chemical composition. This is a disadvantage inasmuch as it may be desirable for the camshaft tube and the cam lobes to have substantially different physical properties so as to optimally withstand the significantly different mechanical forces experienced by the several components.
To address the deficiencies inherent in the casting and forging methods of camshaft fabrication, more recently camshafts have been fabricated by separately producing the camshaft tube and the cam lobes and then installing the cam lobes onto the outer surface of the camshaft tube at desired locations. In the case of the camshaft 10 of FIG. 1, for example, individual cam lobes 12 having a configuration as generally shown in FIG. 2 may be separately fabricated and then positioned about the camshaft tube 11. The components are assembled by disposing the camshaft tube 11 through the bore 17 in each cam lobe, and then affixing the cam lobes 12 to the outer surface of the camshaft tube 11 in desired axial positions and angular orientations. This fabrication method provides greater flexibility relative to the casting and forging methods, and the materials from which the camshaft tube, the cam lobes, and the other components installed on the camshaft are constructed may differ. For example, the cam lobes may be produced from a material particularly resistant to thermal stress and repetitive contact fatigue, while the camshaft tube may be produced from less expensive material such as a machined mild steel.
Fabricating camshafts by installing separately produced cam lobes onto a camshaft tube has the inherent disadvantage that the cam lobes and other camshaft elements must be precisely positioned when affixed. Also, the surface of each cam lobe must have the required shape and dimensions so that it is precisely oriented relative to each other cam lobe surface and relative to the camshaft tube. Accordingly, when fabricating camshafts in this manner, after the cam lobes are installed on the camshaft tube it is common to grind the surfaces of the cam lobes using one or more grinding steps so as to adjust each cam lobe profile and, possibly, adjust the relative angular orientations of the several cam lobes. In an alternate process, the internal diameter of each cam lobe is ground to adjust the tolerances that will be achieved once the cam lobes have been installed on the camshaft tube. Grinding the internal diameters or exterior surfaces of the cam lobes in a precise manner is problematic and, for example, accurately grinding cam lobes produced from powder metal material can be complex and expensive. More generally, grinding of cam lobes requires the use of expensive grinding machines, is a relatively slow process, and requires expensive grinding wheels that must be replaced often.
Accordingly, a need exists for a method of fabricating camshafts wherein cam lobes produced separately from the camshaft tube may be affixed to the exterior surface of the camshaft tube at precise axial positions and angular orientations, and then the cam lobes may be adjusted to proper shape and/or dimensions without employing a grinding operation. More generally, the inventor perceives that a need exists for a method of attaching first and second members wherein, subsequently, the shape and/or dimensions of at least one of the members may be adjusted by a technique other than grinding.
The present invention addresses the above-described needs by providing a method of conveniently attaching and adjusting first and second members. The method includes providing a first member having an outer surface and a second member. The second member is affixed to the outer surface of the first member at a desired axial position along the length of the first member and in a desired angular orientation. The second member may be affixed to the first member using any suitable known technique. Such affixation techniques include, for example, shrink fitting, interference fitting, welding, mechanical or chemical fusing, or any other technique for permanently affixing one member onto a surface of another member at a predetermined position and orientation. Subsequent to affixing the second member to the first member, the shape and/or dimensions of the second member is adjusted by increasing the density of at least a region of the second member. The densification may be accomplished by, for example, a mechanical working technique such roller burnishing, coining, sizing, shot peening, or laser impacting.
According to one embodiment of the present invention, the second member includes an aperture that is sized so that the first member may be disposed within the aperture when the second member is at a temperature greater than a predetermined temperature. The second member is heated to a temperature above the predetermined temperature, and the first and second members are then positioned so that the first member is disposed within the aperture of the second member at a desired axial position and angular orientation relative to the first member. The second member is then cooled to a temperature below the predetermined temperature to securely affix the second member to the first member in the desired axial position and angular orientation. The shape and/or dimensions of at least a region of the second member is then appropriately adjusted by increasing the density of at least a region of the second member. As just noted, the densification may be provided by mechanically working a surface of the second member.
The method of the present invention is particularly suited to the fabrication of camshafts and may be applied to attach cam lobes, sprockets, bearing races, gears and other camshaft elements to a camshaft tube. The camshaft tube includes an outer surface that may be, for example, cylindrical or of any alternate, non-cylindrical configuration adapted to the intended use of the camshaft. When the present invention is adapted as a method of fabricating camshafts, at least one cam lobe is provided. The cam lobe is affixed to the outer surface of the camshaft tube at a predetermined axial position and angular orientation. The cam lobe may be affixed to the camshaft tube using any known technique suitable for permanently affixing one metallic member onto a surface of another metallic member at a predetermined position and orientation. Such techniques include, for example, interference fitting and shrink fitting. Other affixation techniques include known techniques of fusing metallic members together by a welding or sintering operation. Once the cam lobe is appropriately affixed to the outer surface of the camshaft tube, the shape and/or dimensions of at least a surface region of the cam lobe is adjusted by densifying at least a region of the cam lobe. The densifying operation may be accomplished by a mechanical working technique such as roller burnishing, sizing, coining, shot peening, and laser impacting, or by any other densification method suitable for adjusting the tolerance of the cam lobe once assembled onto the camshaft tube.
In embodiments of the camshaft fabrication method wherein the cam lobe is affixed to the camshaft tube using a shrink fitting technique, the cam lobe is provided with an aperture therethrough sized so that when the cam lobe is heated above a predetermined temperature the aperture expands and the camshaft tube may be disposed therethrough. The cam lobe is heated to a temperature above the predetermined temperature, and the cam lobe and camshaft tube are then positioned so that the camshaft tube is disposed through the aperture and is at a desired axial position and angular orientation relative to the camshaft tube. The cam lobe is then cooled to reduce the size of the aperture and to securely engage the external surface of the camshaft tube and fix the position of the cam lobe relative to the tube. Once the cam lobe has been affixed to the camshaft tube in this way, at least a region of the cam lobe is densified by, for example, a roller burnishing, sizing, coining, shot peening, or laser impacting technique, or another suitable mechanical working technique. The densification step is carried out so as to appropriately adjusts the shape and/or dimensions of at least a region of the outer surface of the cam lobe to meet required tolerances.
Roller burnishing is a cold mechanical working technique known to those of ordinary skill in the art and consists generally of applying pressure to the part surface by rolling. More particularly, roller burnishing has been applied in other applications to adjust the dimensions of components formed of powder metal and other metallic materials. To the knowledge of the present inventor, however, roller burnishing has not been applied as a means to adjust the tolerances of cam lobes or other components installed on the camshaft tube of an assembled camshaft. When applied to powder metal parts, roller burnishing also may work to harden the surface of the part. Roller burnishing and other mechanical working techniques are not a metal removal technique. Thus, they differ fundamentally from grinding techniques, wherein the shape and/or dimensions of a part are adjusted by removing material from a surface of the part.
In the method of the present invention, any region of the outer surface of the one or more cam lobes of the assembled camshaft may be roller burnished or otherwise mechanically worked or densified to adjust part shape and/or dimensions. In this way, the outer surface of the one or more cam lobes of the camshaft will properly engage the associated moveable member when the camshaft is assembled into a combustion engine or other device. Moreover, roller burnishing or other densification techniques may be employed to adjust the shape and/or dimensions of other components once affixed in proper positions about the camshaft tube using the method of the present invention. Such other components may include, for example, gears, bearing races, and sprockets.
Cam lobes employed in the method of the present invention may be composed of any material having properties suitable for that application. The method of the present invention for fabricating camshafts is, however, particularly suited to the assembly of camshafts from cam lobes formed from metallurgical powders by press-and-sinter techniques. Such cam lobes preferably are produced from a powder metal material that is resistant to wear and rolling contact fatigue, and that may be roller burnished or otherwise densified or mechanically worked to appropriately adjust the shape and/or dimensions of the surface of the cam lobes. The present inventor has found that cam lobes having the following preferred composition, on a weight percentage basis, provide excellent rolling contact fatigue and wear resistance properties: about 90% to about 99.7% iron, 0 to about 2.0% nickel, 0 to about 1.0% molybdenum, about 0.3% to about 2.0% carbon, 0 to about 3.0% copper, and 0 to about 3.0% chromium. It is noted that compositions that are not strictly within the foregoing compositional ranges also may be used to prepare cam lobes according to the present invention. Additional materials that may be used in the production of cam lobes according to the method of the present invention will be apparent to those of ordinary skill in the art on considering the present description of the invention. Thus, the foregoing preferred metal powder composition should not be considered to limit the scope of the present invention.
The reader will appreciate the foregoing details of the present invention, as well as others, upon consideration of the following detailed description of embodiments of the invention. The reader also may comprehend such additional details upon using the invention.